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Friday, 21 February 2014

There's a whole lot of BITXing goin' on


I've been fiddling some more with my BITX40. I've nearly completed the exciter board and have been playing with the set up of the BFO and the crystal filter. This is where a spectrum analyser really comes into it's own.

Initially I modified the BFO and removed the inductor - the purpose of the inductor is to lower the BFO below the crystal frequency - this is needed in the BITX20 as we need to retain the USB. As I want to keep the LSB in my 7MHz radio, I need the BFO to be higher than in an USB rig.

Here's the carrier or BFO output (in purple) on top of the save I did of the crystal filter sweep - now I don't much like the dip in the top of the Filter sweep, but I can come back to that later.

Now, here the BFO adjustment is at it's lowest frequency and I looked very much to me like this wasn't low enough. Here is the same thing with 1KHz of audio mixed in too:

So, to move the BFO down a bit I have added in the 15pf capacitor in the original design which is in parallel with the trimmer adjustment of the carrier:

Now, this looks close to perfect to me.

So I adjusted the balanced modulator to get as much carrier suppression as I could:

I think this could be way better if I actually matched some diodes, but I don't think it's going to matter a whole pile. Now if I look at the signal on the output side of the crystal filter with all these adjustments made it looks like this:

Which I reckon is as near to perfect as I am going to get.

Here's a recent picture of the board:

So now I need to move and test further into the circuit. We now need to take this 10MHz output of the filter and mix it with my VFO to make the 7MHz LSB signal I am after. So here is the output of the Band Pass Filter in TX:

and here on the 'scope:

Looking like a very nice clean signal at 7MHz to me!

Good, egh?

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